Posted
by
kdawsonon Sunday January 10, 2010 @08:59PM
from the flood-next-time dept.

Hugh Pickens writes "Space.com has a piece about changing theories of planet migration. The classic picture suggests that planets like Earth should have plummeted into the sun while they were still planetesimals, asteroid-sized building blocks that eventually collide to form full-fledged planets. 'Well, this contradicts basic observational evidence, like We. Are. Here,' says astronomer Moredecai-Mark Mac Low. Researchers investigating this discrepancy came up with a new model that explains how planets can migrate as they're forming and still avoid a fiery premature death. One problem with the classic view of planet formation and migration is that it assumes that the temperature of the protoplanetary disk around a star is constant across its whole span. It turns out that portions of the disk are opaque and so cannot cool quickly by radiating heat out to space. So in the new model, temperature differences in the space around the sun, 4.6 billion years ago, caused Earth to migrate outward as much as gravity was trying to pull it inward, and so the fledgling world found equilibrium in its current, habitable, orbit. 'We are trying to understand how planets interact with the gas disks from which they form as the disk evolves over its lifetime,' adds Mac Low. 'We show that the planetoids from which the Earth formed can survive their immersion in the gas disk without falling into the Sun.'"

For me the most amazing aspect of planetary migration is the probable exchange of order for Neptune and Uranus, with Neptune being thrown out to the position of outer planet; without it being ejected from the system, plunging into the Sun or colliding with other big body. Though who knows, perhaps some planet was doomed that way; certainly wild axial tilt of Uranus isn't a testament of calm times.

Gravitational influence of Jupiter; Roche limit - NO!!! That's a very specific term, dealing with tidal forces when bodies get very near. In case of Jupiter & asteroid belt it was more about orbital resonances & energy transfer.

You may want to contact the manufacturer of your targeting system. It appears the target selection queue order has been accidentally reversed. Hopefully they have an update and you can finally get your kaboom.

Not exactly. The body that caused formation of the Moon likely formed in Earth L4 or L5 point; technically making it not a planet. Coming from there also gives less chance for axial tilt such wild as in the case of Uranus...

Since it was already gravitationally bound with Earth, I don't think it changed its orbit in significant way.

Destiny doesn't really factor into it. What we're learning is that essentially our planet is rare. Rocky planet of about the right size, at about the right distance, where our planet didn't fall into the sun, nor did a gas giant falling inwards destroy us, and with a very large moon serving to stabilize the planet's wobble.

All those things coming together for our perfect scenario seem like being very, very against the odds, but the reality is that there's an effing huge number of stars in the universe, and repeat their formation process enough times and you're bound to get our scenario play out from time to time (it obviously happened here or we wouldn't be here).

Only downside is that with all these specific things we're learning that make Earth like planets so rare, it may just be the case that such planets are rare enough that we might as well be the only one. The reality is that if they were rare enough that there were only say, 1 such planet per galaxy, then while the universe itself would be pretty much swimming in Earth-like planets (billions of them), but we'd never be able to detect them, much less contact any possible civilizations on them.

I just want to point out 1 more important factor in contacting, or meeting other civilizations in the universe: Time.

The age of our sun is a blink of an eye in the cosmological time scale. It's like tiny little lightbulbs going on and off and on and off. We might not reach an "on" one before ours turns "off", the destination is simply not turned on yet. It's a very lonely picture, but highly probable.

The sun is around 4.5 billion years old, the universe as a whole - 14 billion years. So the sun has been around about 1/3 of the entire history of the universe. Human history, on the other hand, truly is an eyeblink when compared to the age of the universe, so your overall point may well be valid. We just don't really know how long human history will last, as it isn't quite over yet (Fukuyama [wikipedia.org] notwithstanding).

On the other hand, it is estimated that the universe will have stars in it for around 100 trillion years. Given that, the lifetime of a star like ours really is a blink of an eye, it's just that we will be one of the first ones to blink.

The age of our sun is a blink of an eye in the cosmological time scale. It's like tiny little lightbulbs going on and off and on and off. We might not reach an "on" one before ours turns "off", the destination is simply not turned on yet. It's a very lonely picture, but highly probable.

Seeing how the development of life in general, and our technology in particular, seems to follow an exponential curve, I don't think that Sun dying really has much to do with that probability, unless it's going to die next m

Destiny doesn't really factor into it. What we're learning is that essentially our planet is rare. Rocky planet of about the right size, at about the right distance, where our planet didn't fall into the sun, nor did a gas giant falling inwards destroy us, and with a very large moon serving to stabilize the planet's wobble.

Are we learning that?

I thought things were heading in the opposite direction. Considering that we've been finding exoplanets basically as fast as our capability allows, and every time we

We know basically this: gas giants will form outside the orbits of rocky planets. The star is going to blow the gas outwards and so a gradient is established.

Now, from what we've been seeing, a huge portion of the planetary systems consist of one or more "hot Jupiters". Massive gas giants orbiting extremely close to their parent star. They almost certainly had to have formed farther out and migrated inwards - their orbits might not even be st

Now, from what we've been seeing, a huge portion of the planetary systems consist of one or more "hot Jupiters". Massive gas giants orbiting extremely close to their parent star.

You mean a huge portion of planets we've found, and the reason for that is because they are by far the easiest exoplanets to find -- massive planets close to their sun create the most obvious wobble in the star and the shortest period over which to see it. These are the first exoplanets we were able to find, and we've been looking

Mindless violence? To a modern reader, it may seem like that - but if you live in a society where blood vengeance is sworn by any survivors, you kind of have to wipe out any people group you attack.... and from a purely human perspective (which would appear to be your own perspective), you kind of have to attack someone if you are fleeing THE aggressive super-power of the era and have to go through antagonistic locals to escape the super-power. Compare it to the infighting in Africa within countries... peo

I wasn't aware of sex playing any major role in the Psalms, which are holy liturgical songs, though admittedly I've only read about half of them. I believe you are thinking of the erotic Song of Solomon, various sexual imagery in the prophesies of Ezekiel and Hosea, and historical/mythological narrative in Genesis, 2 Samuel, etc.

Yeah, I guess we really need a way to let people mark their own stuff as funny or not. Then the mods can give ratings like "not very funny" "didn't get the joke" etc. In this case, lacking a smiley, it's hard to see how most people would guess it was a joke.

This would seem to suggest the inner planets formed first and swept the disk of hard derbies, leaving nothing but the gas, which was ultimately blown outward by the pressure of the sun as the disk was swept clear of big chunks.

The gas giants would accumulate at a much slower rate, and almost by definition must be far younger than the rocky planets.

Then there are the oddball moons of the outer planets. Captured planetoids forming late, almost falling into the sun because the disk was pretty much cleared by that time, but being slung outward and captured by chance?

If I'm reading the article right, it says that the gravity of a gas/rock disk around a star will cause the whole thing to migrate inward until it is consumed by the sun. However, account for temperature differences due to varying cooling rates across the disk, then this causes a different force which can be shown to balance out the inward migration.

My question is. Why does the gravitational effects of a gas disk around a star cause inward migration? The only thing I would expect to cause inward migration would be friction resulting in the loss of kinetic energy. I haven't the foggiest idea how a temperature gradient can cause matter to climb out of a gravity well. Maybe I should go looking for the original paper.

Why does the gravitational effects of a gas disk around a star cause inward migration?

Throw a ball up... it comes down. This is gravity. The "base state" for gravity is everything sticking in the centre. Now when something has the right velocity this acceleration towards the centre just causes it to form an orbit around the body.

However given that gasses expand to fill up available space its very hard to have a stable orbit of gas moving at a constant velocity and thus obtaining an orbit. Gasses just do

"Throw a ball up... it comes down. This is gravity. The "base state" for gravity is everything sticking in the centre."

Nope. Your ball analogy doesn't work here. Things in orbit STAY in orbit unless they somehow lose all of their kinetic energy. A ball behaves differently because it NEVER gains enough energy for an orbit. The article says it is the interaction between the cloud and the proto-planet that causes the proto-planets to migrate towards the sun.

"We show that the planetoids from which the Earth formed can survive their immersion in the gas disk without falling into the Sun."

I can understand this part. But the article also says that a gas disk with varying temperatures would cause certain orbits to migrate outwards instead of inwards and THIS

Things in orbit STAY in orbit unless they somehow lose all of their kinetic energy.

Nope, they have to have ENOUGH velocity (Kinetic energy is about the energy required to get it to a given speed) at the right angle in order to counteract the acceleration of the object towards the planet. If the velocity (a vector) isn't right then it will either move out of the orbit into a further orbit (or even escape) if it is too fast or it will fall towards the planet if too slow (as inner orbits require faster veloci

"Nope, they have to have ENOUGH velocity"
Yes. That's why I said things in orbit (meaning they already have an appropriate amount of speed in the right vector) stay in orbit... I didn't say random objects in the solar system stay in orbit.

I can understand this part. But the article also says that a gas disk with varying temperatures would cause certain orbits to migrate outwards instead of inwards and THIS is why proto-planets can survive. But it doesn't say how a temperature gradient can cause migration.

My guess is that there's some sort of considerable net light pressure away from the star. Not acting directly on the planet, but on the gas cloud. What's probably different is that in old models, the light pressure acted only on the surface of the gas cloud, while in this model, due to the temperature gradient, you have light pressure much deeper in the cloud. This means the gas cloud is experiencing net force away from the star throughout a considerable portion of the cloud. That'll help keep planetoids fr

But the article also says that a gas disk with varying temperatures would cause certain orbits to migrate outwards instead of inwards and THIS is why proto-planets can survive. But it doesn't say how a temperature gradient can cause migration.

I didn't read the whole article - far too mathematically dense - but I did get this understanding of the matter:Planetesimals orbit under essentially the rules of Kepler ; this establishes a baseline of velocities for comparison.A gas disk with a uniform temperature wi

I haven't the foggiest idea how a temperature gradient can cause matter to climb out of a gravity well.

Thermophoresis [wikipedia.org] causes particles in a fluid to move because of a temperature gradient. The similarity parameters (Reynolds / Mach / Knudsen) for a planetesimal in an accretion disk are probably similar to the aerosal particles in air that the wiki article talks about.

Yet, it would appear to have caught on. As such... it's linguistic. evolution. As an adult native speaker of English, what I say that effectively communicates my intended point to my intended audience... is English, particularly so if I am emulated by others. One could even argue that I don't have to be a native speaker, though in this case, it is unlikely that emulation by large audiences would occur, limiting the evolution to a temporary mutation. Yes, I am a fan of descriptive grammars.

According to Science Daily [sciencedaily.com] this was the result of a computer simulation which was designed based on a paper, published last year http://arxiv.org/abs/0909.4552 [arxiv.org] . The simulation was "one-dimensional," which seems curious, and they could only afford to simulate 1,000 years out of the estimated 1,000,000 such a disk is expected to last.

So look for more reports of this sort over the next few years. Still, it looks like a big jump forward for our early-solar-system models.

You misread. The relevant paragraph is, "We used a one-dimensional model for this project," says co-author Wladimir Lyra, a postdoctoral researcher in the Department of Astrophysics at the Museum. "Three dimensional models are so computationally expensive that we could only follow the evolution of disks for about 100 orbits -- about 1,000 years. We want to see what happens over the entire multimillion year lifetime of a disk."

You're right. Oops. That still leaves us wondering what a one-dimensional model of the solar system is like, though. Likewise, one could expect better results over time as people do work out how to do three-dimensional models for longer periods.

Presumably, you'd do your modeling as a slice through the disk. Basically, what you're interested in is the effects at different distances from the sun. Hopefully, you can ignore the part about distance above/below the ecliptic and the actual whizzing around the sun, and just focus on a single radial.

The question is, how many slahsdot readers would it take for the probability of you not being alone becoming non-negligent. Given that, and the rate at which we find new slashdot readers all the time. It only follows that one day someone just like you will be found. but they might have tentacles.

"It turns out that portions of the disk are opaque "
Maybe I'm off my rocker but the way this is stated, it sounds like a fact they observed rather than a model that they created. While this "fact" makes logical sense it is far too often that I see the statement "It turns out..."

Astronomers have announced over 500 extra-solar planets and they have barely begun looking. So there are a lot of processes out there creating planets in spite of hypothetical process which may destroy them.

There is no better way to sum up some of the gaps between theoretical and applied science other than: "This contradicts basic observational evidence, like We. Are. Here." Did the proponents of the "classic" model not notice this minor flaw in their reasoning?

Well then what part of orbital dynamics suggest the inner planets would have crashed into the sun?

After all, accretion would happen mostly from the "back" side (hemisphere opposite the orbital direction). The planetoid wouldn't "catch" anything in its orbit, but would be over taken by things on more elliptic orbits.

Therefore the impacts would be accellerative, and puhs the planetoid to a higher orbit.

So where did the original assumption that they would spiral into the sun come from?

A transfer of angular momentum from one region of the disk to another would cause some section of the disk to migrate toward the sun while another set migrated outward. However, it probably isn't caused by a drag force through the residual gas in the disk as most of it is orbiting the same direction as the debris its self. As for accretion, it depends on the distribution of close encounters with objects in a more elliptical orbit. It's fairly easy for an object in orbit to catch up to an elliptically orbiting body.

So, the writer of the space.com article got a wee bit confused, understandably so given that it's quite a tricky topic.

The orbital migration is driven by three effects, one of which was neglected in the original calculations showing inspiral. The main one that was treated was the *imbalance* in the shapes of the spiral arms produced in the disk gas by the orbiting planet. Each spiral arm exerts a gravitational torque on the planet, and the negative torque (removing angular momentum, causing inward migrati

>Well then what part of orbital dynamics suggest the inner planets would have crashed into the sun?

Nothing. According to that theory, everything always gets sucked into everything else, and the universe would be one giant star. Obviously that's not the case, so anyone operating under that theory has a screw loose.

>Therefore the impacts would be accellerative, and puhs the planetoid to a higher orbit.

They don't need to. We could have started from a higher orbit and fallen inward to where we

After all, accretion would happen mostly from the "back" side (hemisphere opposite the orbital direction).

Not really. Simulations show that the accretion happens pretty much symmetrically from both sides.

The planetoid wouldn't "catch" anything in its orbit, but would be over taken by things on more elliptic orbits.

In its precise orbit, no. But from nearby circular orbits? Yes. And the planets tend to feed on stuff from nearby like that. (They definitely have access, where is chance strikes from elliptical orbits are harder to engineer.)

Actually, no. Original poster is right, the gas in the disk orbits slightly slower than the solids do. So there is drag. However, the gas is pretty tenuous, so the drag only really affects things that are small, say less than a meter or so. (Or so classical theory has argued.)